The present invention relates to a device for generating an electric signal in response to movement in a certain direction of an object, eg. a thread, past a predetermined point and/or within a predetermined area.
Within the technology for contact-free sensing of the movement of an object, eg. a thread, past a predetermined point or within a predetermined area, it is known to utilize optical arrangements with a transmitter (light source) and a receiver (light sensitive element) or one transmitter and two receivers. The commonest design is with one transmitter and one receiver. In the case of one transmitter and two receivers, the two receivers are often coupled differentially for compensation against disturbances from the surroundings. In practice however, these latter systems suffer from a major drawback in that they are relatively insensitive, in particular to relatively rapid movement of a thin thread past the receivers. It has moreover been found that such a system does not react to a thick thread moving slowly past the receivers. This may be because the thread bends powerfully and acts on both receivers simultaneously and is, therefore, discriminated against by the differentially coupled receivers. In other systems with two receivers, the object is not to provide a differential coupling but to distinguish between passing dust particles and a thread movement. In this case, an arrangement is employed in which the thread always actuates first one receiver and then the other receiver, while passing dust particles in all probability actuate only the one or the other receiver. In this case, the receivers are coupled to a signal processing circuit which requires first a signal from the first receiver and, within a certain time period, a signal from the second receiver. This otherwise satisfactory solution does not, however, allow for a strongly bending thread, since such a thread can actuate both receivers simultaneously, as mentioned above.